Synesthesia is a concept that has always fascinated people. People have long since claimed to process colors as sounds, or to associate colors with music or words.

But how can we prove that synesthesia actually exists? And what happens when someone associates particular words with tastes?

Top image: Zametalov/Shutterstock.

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Synesthesia, as a concept, was not always believed to be an actual phenomenon, as opposed to a thing that people talked themselves into feeling. Although plenty of people reported colors associated with music or words, or processed colors as sounds, their experiences were attributed to an over-active imagination or desire for attention. Even those who believed that wires might be crossed in the brain didn't think the phenomenon could be studied. There was no way to check, after all, what was in a person's head.

Eventually, people built on established scientific checks meant to test perception. One of the earliest checks for a common kind of synesthesia developed from a test for color blindness. A large percentage of the people in the world are red-green color blind. Show them a red number on a green background and, as long as the background and the number are equally dark, they won't be able to see the number while anyone else can see it as plain as day.

Doctors reversed this test to check for one of the most common types of synesthesia — seeing different characters as different colors. A densely cross-hatched group of lines were put on a card. On top of these lines were drawn numbers or letters. Both the lines and the numerals or letters were the same color and thickness, and control test subjects just saw dark lines interlocking. Some of them eventually picked out the letter. The synesthesiacs saw the characters right away, as easily as most people would see a green number on a red grid. Another popular test shows jumbled 2s and 5s. The ordinary viewer can't pick them out, but they show up to people who associate the different numbers with colors.

Other tests were less romantic. Scientists simply asked test subjects what color each number or word evoked. They sent the subjects away and called them back in six months, with no notice. If the colors associated with words or numbers changed, it was likely to be imagination. If most of them stayed the same, the test subject really did have synesthesia.

This test proved helpful with one of the more unfortunate types of synesthesia, lexical-gustatory synesthesia. While most types of synesthesia are poetic — like hearing a symphony when seeing a painting, or associating a concert with a beautiful light show — this is more of an inconvenience. People with lexical-gustatory synesthesia taste many different types of words.

Often these people repeat their reports of tastes, years after their initial interview and tests. One woman had a list of words she couldn't stand because of the terrible tastes associated with them - Cincinnati, number, portable, squirm, and phony. Another test showed that synesthesia might go even deeper than words. When lexical-gustatory people were shown images, they sometimes got taste sensations even when they couldn't identify what the image was. (A phonograph made one woman taste Dutch chocolate. The taste was repeated when the scientists called her months later.)

Which isn't to say that people aren't influenced by the sound of the words. Researchers noticed that words with 'm' sounds trigger mint flavors, words with 'aye' sounds taste like bacon, and words with 'x' sounds taste like eggs. Also, food names tend to taste like themselves. (Never talk about stewed greens during dessert with one of these people.)

What's interesting is, in word-associative synesthesia, most of the people who have it are to some extent influenced by the language that they speak. People who have this different view of the world may be able to change it by learning new languages. If English means too many unbearable tastes or unpleasant colors, try French. Their food is usually better, anyway.